Telecommunications networks are used to provide large scale revenue generating services to both residential and business subscribers. Services provided to these subscribers have migrated over the past ten years from being primary voice communications, often referred to as POTS or Plain Old Telephony Services, to a wider range of services based on packet transmission to interconnect more computationally sophisticated terminal devices and service platforms. Subscriber bandwidth delivery has substantially increased and continues to do so such that when the infrastructure will support it, carriers will be technically able to offer all key consumer services over a single packet based network structure.
Evolved technology has permitted increased packet based bandwidth delivery to the subscriber premises, and sophisticated L3 and above protocols and content management and control architectures have been developed for use in the core, but the infrastructure linking the edge to the core is not ideal for supporting the growth caused by integrated service delivery. This is true for unicast application to person services, such as IPTV, but is more acutely so for broadband person to person peering services such as video exchange and interactive gaming. This is illustrated by considering the carrier network as being divided into three zones. The first zone is the connection between the packet based service edge device and the subscriber. This is known as the access network. The second zone is the connection between the packet based service edge devices and the core network devices. This is known as the collector network. The third zone is the core, which provides connection between collector networks.
Current technology in the collector provides interconnection between the packet service edge devices and core devices using fixed DWDM optical channels. These are provided at the service edge as bi-directional standard packet interfaces operating at high bandwidth. An example of this type of interface is 10 Gbit Ethernet. As the optical channel has fixed capacity, network planning dictates that the occupancy of the channel is less than the full capacity, to permit the smooth flow of packets in combined baseband with superimposed burst peaks. As the access network bandwidth increases, additional optical ports are required to provide connectivity back to the core. The result of this is that the service edge device interfaces are transferred to the core of the network, where they need to be switched for grooming and aggregation purposes before handoff to the core devices. A problem with this arrangement is that with increasing uptake of services, there is an increase in partially filled optical DWDM channels through the collector and a corresponding rise in switching ports and fabric capacity at the collector head end.
Distributed packet switches based on burst optical technology have been described in two broad categories. These are open optical systems and closed optical systems. An open optical system is one that provides external optical connections. A closed optical system permits the use of a deterministic optical power management control system. This means that there are no external optical interfaces on the line side. All of the external interfaces to this invention are on the port side. Port side interfaces may be on optical fibers, but no interfaces to the optical switching and transmission system are provided externally. Distributed packet switching based on closed optical systems has been described in two categories. These are synchronous and asynchronous systems. With the former precise timing is distributed around the closed optical system and this is used to ensure that each port on the distributed switch can be given deterministic access to the available capacity of the fiber in conjunction with other ports also requiring access. This process is known as scheduling.
However, a shortcoming of a synchronous system is the complexity of precise timing, which leads to the impractical need to control the inter node fiber lengths, and the inefficiency resulting from the misalignment of packet lengths to transmission container unit lengths available in the optical system.
Distributed packet switches based on asynchronous burst optical switching has been previously described in PCT patent publication number WO2005/125264. In these systems a means of collision avoidance is provided such that a port on the system can detect an optical channel is free, switch the source laser to the free channel and transmit a burst of packet data. A delay is provided such that if the channel is subsequently detected as being used by an upstream node, the transmission can be truncated and a collision is avoided. This system offers efficient use of the optical medium, responsive access to the optical medium at any local port, and does not have any impractical restrictions on fiber length or precise timing complexities.
However, a drawback of a distributed switch with asynchronous access with collision avoidance technology is that when the network is heavily loaded with flows of packets, an optical upstream channel can secure an optical channel for its needs and hold on to it in response to loading demands thus blocking downstream nodes from gaining access.
While this is not problematic per se in fully meshed flows of traffic, it is unusable in the collector application where a large proportion of the flows of packets are converging on optically downstream nodes. To overcome this, methods have been proposed where a feed back mechanism arbitrates access to an optical channel from an oversubscribed destination. However this simple single dimensional approach gives rise to lock up of wavelength selection at the source nodes, where each node can only access its furthest neighbor with the only way out to reset, or oscillations occur, where the depth of occupancy of input data buffers alternatively drains and fills. With both of these conditions latency and jitter are induced into services carried through the distributed switch rendering it impractical for real networking applications.